The present invention generally relates to systems for navigation. In particular, the invention relates to correcting angular speed, offset error and gain error in a system for navigation.
Systems in the art for navigating a vehicle may employ a gyroscopic sensor to detect the angular speed of the vehicle. It is well known that each such gyroscope has several characteristics whose actual values are specific to that particular gyroscope. These gyroscopic characteristics include offset error for the typifying offset; gain error for the typifying gain; and right and left gain differences for the typifying right and left gain. These gyroscopic characteristics cause the navigation apparatus to incorrectly position the vehicle. Each is described in turn below.
Each gyroscope gives a voltage output corresponding to the angular speed of the object (e.g., the vehicle) on which it is mounted. FIGS. 2A and 2B illustrate the relationship of the gyroscopic output voltage to the angular speed of the vehicle. The angular speed (AS) of the vehicle equals the output voltage of the gyroscope (V.sub.GYRO) less the typifying offset voltage (V.sub.OFFSET), divided by the gain (G): ##EQU1##
However, each gyroscope has a characteristic offset error. FIG. 3 illustrates the relationship of two offset errors to the typifying offset of the gyroscope. In FIG. 3, the offset error 310 is positive, causing the high offset 320. The offset error 311 is negative, causing the low offset 321.
Accordingly, the angular speed of the vehicle is better determined not as in equation (1) but as in equations (2) and (3): ##EQU2##
The angular speed with the offset error (AS.sub.OFFSET ERROR) equals the voltage output of the gyroscope with the offset error (V.sub.GYRO, OFFSET ERROR) less the offset (V.sub.OFFSET), divided by the gain (G). The voltage output including the offset error (V.sub.GYRO, OFFSET ERROR) is sum of the voltage output of the gyroscope (V.sub.GYRO) and the offset error (V.sub.OFFSET ERROR).
Thus, even though the vehicle may be moving in a straight line, the systems of the prior art may determine that the vehicle is turning right when the characteristic offset error is positive. Conversely, the systems of the prior art may determine that the vehicle is turning left when the characteristic offset error is negative. These wrong determinations cause systems of the prior art to position the vehicle incorrectly.
Further, each gyroscope has a characteristic gain error. FIG. 4 illustrates the relationship of two gain errors to the typifying gain of the gyroscope. A high gain error is charted as the high gain error 410. A low gain error is charted as the low gain error 411. As a result, in the presence of gain error, the angular speed of the vehicle is determined by equations (4), (5) and (6) rather than equation (1): ##EQU3##
Now the angular speed in consideration of the gain error (AS.sub.G ERROR) is the difference between the output voltage of the gyroscope (V.sub.GYRO) and the offset voltage (V.sub.OFFSET), divided by the gain with gain error (G.sub.G ERROR). The gain with gain error (G.sub.G ERROR) is the typifying gain of the gyroscope (G) ratioed by a factor K.sub.G ERROR. K.sub.G ERROR equals 1 when there is no gain error but is less than one when the gain is erroneously low. K.sub.G ERROR is greater than one when the gain is erroneously high.
The presence of the characteristic gain error causes apparatus of the prior art to detect and determine the position of the vehicle incorrectly. For example, where the vehicle actually turns 90.degree., a system with a high gain error determines that the vehicle has turned more than the actual 90.degree.. Likewise, a system with a low gain error detects that the vehicle has turned something less than the true 90.degree..
As a final example consider the difference in characteristic gyroscopic gain error between right and left vehicle movements. FIG. 4 illustrates the relationship of the differential right and left gain error with respect to the typifying gain of a gyroscope. For a vehicle turning m degrees/sec to the left and then m degrees/sec to the right, a system not adjusting for the differential right and left gain error detects different absolute values .linevert split.n'.linevert split. and .linevert split.n".linevert split. for the left and right turns, respectively.
Accordingly, there is a need for a methodology and device which detects a characteristic error in a gyroscope, including offset error, gain error and the differential right and left gain error.
Also, there is a need for a methodology and device which compensates for a characteristic error in a gyroscope, including offset error, gain error and the differential right and left gain error.
In U.S. Pat. No. 5,404,307 (April, 1995), Odagawa describes a navigation apparatus which corrects data from a angular speed detector with data from an azimuth detector. Likewise, in U.S. Pat. No. 5,424,953 (June, 1995), Masumoto et al. also describes a navigation apparatus which corrects data from an angular speed detector with data from an azimuth detector.
However, when the gyroscope has different gains for right and left turns, the Odagawa navigation system cannot effect corrections so that the gain correction value is accurate for both right and left turns.
Further, Masumoto attempts to detect the straight movement of a vehicle using the output of an angular speed sensor and a direction detection sensor. However, where the angular speed sensor has an offset error, the Masumoto device operates incorrectly.
In both the Odagawa and Masumoto devices, before the first determination of the gain error correction value after powering on of the system, compensation for that gain error is not possible.